1. Field of the Invention
The present invention relates to a scrubber and an exhaust gas treatment apparatus for removing dust from an exhaust gas, and more particularly to a scrubber for removing dust from an exhaust gas at a high efficiency, and an exhaust gas treatment apparatus having such a scrubber, a heat exchanger disposed at the downstream side of the scrubber, and other equipment.
2. Description of the Related Art
In a semiconductor fabrication process and a liquid crystal panel fabrication process, an exhaust gas containing silane gas (SiH4) or halogen gas (NF3, ClF3, SF6, CHF3, C2F6, CF4, or the like) is discharged from fabrication apparatuses in the semiconductor fabrication process and the liquid crystal panel fabrication process. Since the silane and halogen gases are harmful, combustible, or hardly decomposable, the exhaust gas containing such silane and halogen gases cannot be released to the atmosphere as it is. Therefore, it has been customary to use an exhaust gas treatment system in which the exhaust gas is introduced therein and treated to be harmless, and is then released to the atmosphere.
A conventional exhaust gas treatment system is shown in FIG. 16 of the accompanying drawings. As shown in FIG. 16, the conventional exhaust gas treatment system comprises an exhaust gas treatment apparatus 61, and an exhaust gas treatment apparatus 72 disposed downstream of the exhaust gas treatment apparatus 61. The exhaust gas discharged from a semiconductor fabrication apparatus or the like is introduced into the exhaust gas treatment apparatus 61 in the direction indicated by the arrow G. The exhaust gas treatment apparatus 61 comprises a burner 63 and a liquid atomizing area 64. The exhaust gas introduced into the burner 63 is combusted by flames 65 generated in the burner 63, and is thus heated, oxidized and decomposed. Thereafter, the exhaust gas is fed to the liquid atomizing area 64, and is cooled by a coolant 67 sprayed from coolant spray nozzles 66. At this time, a part of dust contained in the exhaust gas is adsorbed by the coolant 67, and is then discharged together with the coolant 67 through a U-shaped drain pipe 68 to the outside of the exhaust gas treatment apparatus 61. The drain pipe 68 comprises a U-shaped pipe storing a liquid therein for discharging only the coolant 67 containing dust without allowing the exhaust gas to pass therethrough. The exhaust gas cooled in the liquid atomizing area 64 is fed to the exhaust gas treatment apparatus 72 through an exhaust gas pipe 17.
The exhaust gas treatment apparatus 72 comprises a scrubber 60, and a mist collector 69 connected to the scrubber 60 and disposed downstream of the scrubber 60. The scrubber 60 has a casing 2 and an impeller 3 housed in the casing 2. The impeller 3 has a number of impeller blades, and is fixed to a driving shaft 4 which is coupled to a motor 16. The impeller 3 is thus rotated by the motor 16 at a high speed. The casing 2 has an exhaust gas inlet 8 at a position near the central portion of the impeller 3. A cleaning liquid supply tube 9 extending into the impeller 3 for ejecting a cleaning liquid 10 is provided through the exhaust gas inlet 8.
The exhaust gas which has been treated by the exhaust gas treatment apparatus 61 is drawn through the exhaust gas pipe 17 and the exhaust gas inlet 8 into the central portion of the impeller 3 by the rotation of the impeller 3. At this time, the cleaning liquid 10 is ejected from the cleaning liquid supply tube 9, and the exhaust gas is stirred together with the cleaning liquid 10 by the rotation of the impeller 3. Dust contained in the exhaust gas is adsorbed and captured by the cleaning liquid 10, and is thus removed from the exhaust gas. The dust adsorbed by the cleaning liquid 10 is discharged together with the cleaning liquid 10 through a U-shaped drain pipe 15 to the outside of the exhaust gas treatment apparatus 72. The cleaning liquid 10 primarily comprises water.
The exhaust gas from which dust has been removed is discharged from the scrubber 60 through an exhaust gas outlet 7 provided at an upper end portion of the casing 2. The exhaust gas discharged through the exhaust gas outlet 7 flows into the mist collector 69 disposed at the downstream side of the exhaust gas outlet 7. Mist contained in the exhaust gas is trapped and collected by the mist collector 69, and the exhaust gas from which the mist has been removed is thus finally released to the atmosphere.
In the conventional exhaust gas treatment system having the above structure, the following problems arise:
1) When the exhaust gas containing silane gas (SiH4) or the like is heated, oxidized and decomposed in the exhaust gas treatment apparatus 61, the treated exhaust gas contains fine dust having a diameter of 1 μm or less at a high concentration. If the exhaust gas containing such fine dust is scrubbed by the above conventional scrubber, then the dust is removed by a percentage ranging from 20 to 60%, which is a low dust-removal efficiency.
2) In many cases, the conventional mist collector comprises a filling material (SUS, ceramic, plastic, or the like), a metal mesh, a filter, and the like. Since the mist collector has small openings for passing the exhaust gas therethrough, the mist collector tends to be easily clogged by the mist. Consequently, it is necessary to carry out time-consuming cleaning of the mist collector. Further, because a saturated vapor contained in the exhaust gas cannot be trapped by the mist collector, when a temperature of the exhaust gas is lowered after the exhaust gas is passed through the mist collector, the saturated vapor that has passed through the mist collector is changed into mist, thus causing a pipe provided at the downstream side of the mist collector to be clogged by the mist.
There has been known a treatment method in which a gas, to be treated, such as SiH4 which can easily react with water is directly drawn into the scrubber to be made harmless without passing through an exhaust gas treatment apparatus. However, the gas to be treated and water contained in the cleaning liquid attached to the exhaust gas inlet react with each other to produce SiO2, thus causing the exhaust gas inlet to be clogged.
Further, as shown in FIG. 16, in the case where the scrubber 60 is connected to an apparatus having the drain pipe 68 at the downstream side of such apparatus, the following problem arises:
FIG. 17 is a schematic view illustrating the relationship between a liquid level of the drain pipe and the scrubber shown in FIG. 16. In the arrangement shown in FIG. 17, if a suction pressure generated in the exhaust gas pipe 17 by the rotation of the impeller 3 is excessively high, then the liquid level 68b of the drain pipe 68 rises, and hence the exhaust gas pipe 17 is closed by a liquid 68c. On the other hand, if the suction pressure is too low, then the liquid level 68b of the drain pipe 68 is lowered to cause a liquid sealing state to be lost, resulting in a leakage of a gas, to be treated, through a liquid discharge port 68a of the drain pipe 68.
The above problem can be solved by changing an operating condition of the motor 16, i.e., increasing or decreasing the rotational speed of the impeller 3. However, it is preferable to keep the operating condition (the rotational speed) of the motor 16 unchanged in order to prevent an exhaust gas treatment capability of the scrubber 60 from being lowered. Heretofore, therefore, it has been attempted to either provide a restrictor such as a butterfly valve (not shown) at an exhaust gas outlet side of the scrubber 60 or increase a liquid sealing length of the drain pipe 68 to adjust the liquid level 68a. 
However, in the case where the restrictor is provided at the exhaust gas outlet side, the restrictor tends to be clogged due to deposition of dust thereon. On the other hand, in the case where the liquid sealing length of the drain pipe 68 is increased, a large area is required for the installation. From such viewpoints, it is problematic to employ these methods. Consequently, it has been customary to solve the above problems by increasing or decreasing the rotational speed of the impeller 3. Specifically, when the suction pressure is too high, the rotational speed of the motor 16 is required to be lowered. As a result, the scrubber 60 has to be operated in such a state that the exhaust gas treatment capability of the scrubber 60 is lowered.